ASTERIAD.fi. 75 



side of each arm is a branched ovary, which opens ex- 

 ternally near the union of each pair of arms, though the 

 aperture is seen with difficulty, saving- when the animal is 

 in full egg. On the under surface of the interior of the 

 arms are seen rows of round bladder-like vesicles ; these 

 are the vesicles of the feet, and they are connected with 

 vessels which communicate with a vascular circle surround- 

 ing the mouth. The internal walls of the suckers and 

 their vessels are ciliated, and a continual circulation of the 

 fluid they contain goes on within them. The nervous 

 system has been described by Tiedemann, the great anato- 

 mist of Heidelberg. It is a white circular cord surround- 

 ing the mouth, and sending off branches to the arms. 

 Professor Ehrenberg says these nerves run to the ex- 

 tremity of the arms, and that there is a nervous ganglion 

 under each of the red spots which are seen near the tips 

 of the rays, and which from analogy may be regarded as 

 eyes of low organisation. The existence of ganglions in 

 the nervous system of these animals is, however, generally 

 regarded as doubtful. 



The number Jive is dominant among the Echinodermata, 

 and we accordingly find it regulating the forms and organs 

 of the Starfishes, though numerical variation more fre- 

 quently occurs in this order than among the Ophiurse or 

 Sea-Urchins. In no British genus of true Starfishes is 

 number positively specific, though generic in several. In 

 Luidia and Solaster it is of least importance. The num- 

 bers of variation are generally four and its multiples, which 

 we might expect when we consider that four is the domi- 

 nant number of the parallel order of Actinodermata. 



The genera of Asteriadse are founded on characters 

 derived from the outline of the body, the number of rows 

 of suckers in the avenues, and the structure and arrange- 



